Abstract(#br)Reactive oxygen species (ROS) including singlet oxygen ( 1 O 2 ) and hydroxylradicals (OH) photogenerated in natural waters play important roles in indirect photolysis of man-made pollutants. This study was conducted to investigate how the generation of these two ROS influences the degradation of two highly toxic insecticides (methomyl and carbaryl) in river water. To accomplish this, the reaction rate constants of 1 O 2 and OH with carbaryl and methomyl were determined; the degradation rate constants of the tested insecticides in ultrapure water (direct photolysis) and in river water in the presence and absence of 1 O 2 and OH scavengers were also measured. The rate constants for the reaction of ... name="rad"/>OH with carbaryl and methomyl were found to be (14.8 ± 0.64) × 10 9 and (4.68 ± 0.52) × 10 9 M −1 s −1 , respectively. The reaction rate constant of 1 O 2 with carbaryl (2.98 ± 0.10) × 10 5 M −1 s −1 , was much higher than that of methomyl (<10 4 M −1 s −1 ). Indirect photolysis by OH accounted for 63% and 62%, while 1 O 2 accounted for 26% and 30% and direct photolysis accounted for 1.4% and 7% of methomyl and carbaryl degradation, respectively. The high degradation rate in river water demonstrated by both insecticides suggests that indirect photolysis mediated by OH is an important means of their degradation in river water. In addition, kinetic calculations of OH-mediated degradation rate constants of the compounds agrees with their experimentally-determined values thereby confirming the importance of OH towards their degradation.